1. Field of the Invention
The present invention relates to a transfer belt device and an image forming apparatus provide with the same.
2. Description of the Related Art
A so-called tandem type color printer is an example of a conventionally known image forming apparatus, in which a photosensitive drum for producing a monochrome (black) image and photosensitive drums for producing three color (cyan, magenta, yellow) images are arranged generally in line. For the sake of simplicity in the following discussion of the invention, the photosensitive drum for producing the monochrome image and the photosensitive drums for producing the color images are referred to as the “monochrome photosensitive drum” and “color photosensitive drums”, respectively.
This kind of color printer is provided with an transfer belt device including an endless transfer belt which is so disposed as to contact curved outer surfaces of the aforementioned four photosensitive drums, transfer rollers which are mounted in opposed relation with and are pressed against the corresponding photosensitive drums with the transfer belt passing in between the photosensitive drums and the transfer rollers in order to transfer toner images formed on the photosensitive drums to the transfer belt, and a tension roller which is placed in contact with a surface of the transfer belt exerting specific tension thereon.
Generally in color printers, black toner is used more often compared to color toners. One problem of the conventional color printers is that small quantities of the color toners are consumed even in monochrome print mode, because the color photosensitive drums rotate even during monochrome printing. It has therefore been desired that the photosensitive drums be stopped and kept out of contact with the transfer belt in the monochrome print mode.
One conventional approach to the solution of this problem is described in Japanese Unexamined Patent Publication No. 2002-99132, for example. An arrangement proposed in this Publication is a so-called 3-color removal mechanism built in an image forming apparatus, in which transfer rollers mounted in opposed relation to corresponding color photosensitive drums are made movable toward and away from the color photosensitive drums so that the transfer belt is switched between a state where the transfer belt is positioned in contact with the color photosensitive drums and a state where the transfer belt is positioned apart from the color photosensitive drums.
In the image forming apparatus thus structured, the tension roller is made swingable in a direction perpendicular to an axial direction thereof and, making such swing motion, the tension roller continuously exerts specific tension on the transfer belt even when the transfer belt is displaced as a result of movement of the transfer rollers.
If the longitudinal axis of the tension roller fluctuates due to the swing motion thereof in the image forming apparatus, contact pressure exerted between the tension roller and the transfer belt would vary along the axial direction of the tension roller, causing risk of an image transfer failure.